ladier@aerospace-valley.com 09/2010 Gérard LADIER Airbus / Aerospace Valley Software aspects of aeronautical certification and static analysis tools

Equipment Rules (JAR/FAR 25-1309) “Essential” equipment must be designed to perform its intended functions The airplane systems and associated components, considered separately and in relation to other systems, must be designed so that : The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable, and The occurrence of any other failure conditions which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable ...

Software Means of conformance It is in general not feasible to assess the number or kinds of software errors , if any, that may remain after the completion of system design, development, and test. DO - 178B/ED 12B, provides acceptable means for assessing and controlling the software used to program digital computer based systems ” Software

First principle We can’t get clean water from a dirty pipe Þ We can’t get clean water from a dirty pipe Evidences are requested on the pipe …

DO-178/ED-12 : evidences on the pipe … “DO-178B/ED-12B is primarily a process-oriented document” => Set of requirements on the processes (dev., verif, etc.) and their outputs “The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable” => Evidences on the fulfilment of these requirements vary depending on the software “criticality” level

Life cycle and processes Definition of separate processes that will be combined for a given project to describe its life cycle: Planning process (organization/plans rather than scheduling) Development process (specification, design, coding, integration) Integral processes (verification, configuration management, quality assurance, certification liaison process). Define for each process: The Assurance objectives The means of achieving those objectives The process input data The process activities The process products The transition criteria, which must be met in order to proceed

Main common requirements on the developement processes Standards must be written and evidences of compliance with the rules should be provided Rules ? Dozen of documents Define precisely how to perform an activity (methods, means, constraints, expected outputs, etc. consistent precise tracable Each requirement or design item should be developed verifiable

Configuration management & Quality Assurance To assist in satisfying general objectives to: Control configuration of the software throughout the software life cycle Be able to replicate the executable object code Control process inputs and outputs during the software life cycle Provide baselines for review, assessment and change control Ensure problems management and change control Ensure archiving and recovery. To provide evidences that: What’s done is what’s described in plans Transition criteria are reached A conformity review of the software is conducted Main characteristics : Independence Active role during the life cycle process

Tools qualification -1 Necessary when processes required by the rest of the document are eliminated, reduced or automated by the use of a deterministic software tool whose outputs are not verified. 3 qualification criteria depending on the risk associated to the tool usage : Criteria 1 : Development Tool Criteria 3 : verification tool Criteria 2 : Verification tool which could have an impact on the resulting software : used to justify the elimination or reduction of: Verification process other than that automated by the tool, or Development process which could have an impact on the resulting software

Tools qualification - 2 Combination of the qualification criteria with the software level to give the Tool Qualification Level: Two distinct roles are defined : The user The developper He defines his needs (Tool Operational Requirements-TOR) and validate the tool in its usage context He defines his development specification (Tool Requirement-TR), develops the tool and provides the life cycle data Sharing of activities between these two roles are defined for COTS tools(11.3)

Tools qualification - 3 Qualification requirements : TQL 1, 2 et 3 : TQL 4 ones + « implementation » requirements : Depending on the level of the final product software developed with the tool TQL 4 : TQL 5 ones + «project management» requirements : TOR reviews (complete, accurate, and consistent) Definition of processes in plans Definition of the TR and verification / TOR Verification of the tool / TR et requirements coverage Configuration and change managements TQL 5 : «contracting authority» requirements, mainly concentrated in the TOR : Table T-0 defining all the “user” requirements including the validation of the tool versus the need expressed in the “avionics” PSAC Plus one requirement : «Impact of known problems on the TOR »

Second principe A clean pipe may not deliver clean water Þ Filters are piled to detect and eliminate impurities

Verification Basic principles: The most important section of DO-178/ED-12, in term of : volume : 13 pages of description ( ~ 5 pages for other processes ) Workload incurred (A380 : 4 lines of test for line of embedded code …) Basic principles: “Integral” process => applies to all the development processes Combination of : Reviews , Analysis Tests to detect and identify errors introduced during development

Reviews ? Analysis ? Tests ? Three major tools for bug-busters : Review : inspection of a product by an independant (level A) person; qualitative evaluation Analysis : detailed examination of a process, potentially done by a tool quantitative evaluation Test : running the software and comparison of actual outputs to expected ones Functional test NO TEST BASED ON CODE STRUCTURE Functional & Structural coverage analysis

DO-178/ED-12 - The verification process System Requirements High-Level Software Architecture Source Code Executable Object Code (A-2: 3, 4, 5) (A-2: 7) (A-2: 6) (A-2: 1, 2) Low-Level A-3.2 Accuracy & Consistency A-3.3 HW Compatibility A-3.4 Verifiability A-3.5 Conformance A-3.7 Algorithm Accuracy A-3.1 Compliance A-3.6 Traceability Compliance: with requirements Conformance: with standards A-6.5 Compatible With Target A-6.3 Compliance A-6.4 Robustness A-6.1 Compliance A-6.2 Robustness A7 Verification of verification (Functional & Structural coverage) A-4.1 Compliance A-4.6 Traceability A-4. 8 Architecture Compatibility A-4.9 Consistency A-4.10 HW Compatibility A-4.11 Verifiability A-4.12 Conformance A-4.13 Partition Integrity A-4.2 Accuracy & Consistency A-4.3 HW Compatibility A-4.4 Verifiability A-4.5 Conformance A-4.7 Algorithm Accuracy A-5.1 Compliance A-5.5 Traceability A-5.2 Compliance A-5.3 Verifiability A-5.4 Conformance A-5.6 Accuracy & Consistency A-5. 7 Complete & Correct

Third principe Þ Potentially opposite interests are at stake Independant authorities assess the process and product

The ED-12B/DO-178B - certification liaison Objective: ensure effective communication/understanding between the applicant and the certification authorities Means: The Plan for Software Aspects of Certification, given to the Authorities as early as possible Reviews carried out by the certification authorities “software” specialists as much as they want Software Accomplishment Summary and Software Configuration Index.

Fourth principe Þ All the interests but must be taken into account to build a recognized set of requirements DO-178/ED-12 is built and updated by all the concerned specialists and actors

Consensus on requirements Joint meetings between the RTCA SC-205 EUROCAE WG-71 Openness, consensus : More than 1200 people registered on the WEB site about 120 attendees in each meeting : aircraft manufacturers, engine makers, equipment suppliers, authorities, scientists and consultants The final text has to be agreed by each of the attendees

DO-248C/ED-94C FAQ/DP/RATIONALE Supplement A Supplement B Supplement … ...Supplement N Interface Spec DO-248C/ED-94C FAQ/DP/RATIONALE DO-278A / ED-109A The « core document » will be completed by supplements : Supplement –guidance used in conjunction with DO-178C/ED-12C that addresses the unique nature of a specific technology or a specific method. A supplement adds, deletes or otherwise modifies: objectives, activities, explanatory text, and software life cycle data in DO-178C/ED-12C. This structuring principle enables incremental future development of software requirements Il est difficile de décrire les objectifs de safety qu’un cycle de développement de logiciel doit atteindre, sans être prescriptif sur les moyens. Ex : le test de logiciel est un moyen, pas un objectif, et il est pourtant requis dans le DO-178B Dès qu’on est prescriptif sur les moyens, on se fait un jour ou l’autre rattraper par une évolution technologique qui rend la prescription difficile à suivre. Ex : remplacement du test par des méthodes formelles de vérification sur certains logiciels de l’A380 Pour la version C (fin 2010), il a été décidé de toucher le moins possible au cœur du DO-178/ED-12 mais de lui adjoindre des suppléments pour prendre en compte les évolutions passées et futurs du Génie Logiciel

The DO-178C/ED-12C : evolution of the content Major improvements are in the supplements : Object Oriented technology Model Based Development Formal Method Technology Tools Qualification In the core document : Basically clarification and improvement in consistency and accuracy. Except for the tool qualification part : Separation of the «why ?» part (>in the core doc) from the «how ?» part (>supplement) Significant evolution of the « why ? »

Formal Methods Technology supplement The FM introductory section insists on the interest of using FM for verification The FA (Formal Analysis) may completely replace : Reviews & analysis (except for validation of « derived requirements ») Conformance test versus /HLR et /LLR Software integration tests Robustness tests FA may help for the verification of compatibility with the hardware FA cannot replace HW/SW integration test The structural coverage objectives are achieved if it can be demonstrated that : Each requirement is completely covered The set of requirements is complete in regards of the attended function There is no non expected dependences between output and input data There is no dead code

Þ Fitfh principe Only requirements are mandatory, not the means Building the « pipe » is to be dealt with by the suppliers

Which industrial use for Static Analysis tools ? Today

Not so frequent …

How to convince certification Authorities ? Example of the “unit level proof of LLR” Discussion with Cert. Auth. software specialists much before the actual use of the tool, to get a general feedback (go/no go) Demonstration of the soundness of the approach Definition of specific rules and standards Demonstration of the completeness of the properties Definition of LLR as pseudo-code + properties Smooth integration in the overall verif./traceability processes Detect and eliminate dead code Verify the executable code Qualify the tool (verification ++ tool)

Which industrial use for Static Analysis tools ? Tomorrow …

Tomorrow … 1 Proof of absence of Real Time Execution error : ASTREE (ENS/ABSINT) For effective certification credit Precision of Floating-point calculus : Fluctuat (CEA) : Abstract Interpretation based; analysis of C or assembly code Safe computation of the numerical (rounding) errors introduced by basic operators or input filtering code In use by Airbus for evaluation : Research prototype for the C language Pre-industrial prototype for the TMS320C33 assembly language

Tomorrow … 2 Validation of the compilation Lcertify (ENS) : Research activity in Airbus Compcert (INRIA) : Industrial prototype available Efficient compilation of a complete subset of the A380 fly-by- wire software Other “translation validation tools” would be highly desirable (e.g. Scade -> C) Various analysis of C code : FRAMA-C (CEA) : Plugin WP to succeed CAVEAT Plugin TASTER for syntaxic control (coding rules enforcement) Plugin for data flow/control flow verification, coming soon

Special thanks … To the co-chairs of the fantastic formal methods SG of the DO- 178C joint committee: Kelly Hayhurst (NASA) Duncan Brown (Rolls Royce) To the Airbus Formal Methods dream team : Famantanantsoa Randimbivololona Jean Souyris David Delmas And to Hervé Delseny from Airbus, who liaised both … Made on http://www.wordle.net

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